There has been a great deal of interest in developing better and more efficient methods for storing energy for applications such as radio communication, satellites, portable computers, and electric vehicles to name but a few. Accordingly, there have been recent concerted efforts to develop high energy, cost effective battery cells having improved performance characteristics.
Rechargeable, or secondary cells, are more desirable than primary (non-rechargeable) cells since the associated chemical reactions which take place at the positive and negative electrodes of the battery are reversible. Accordingly, electrodes for secondary cells are capable of being regenerated (i.e. recharged) many times by the application of an electrical charge. Numerous advanced battery systems have been developed for storing electrical charge thereon. Concurrently, much effort has been dedicated the development of electrical battery charging systems adapted to apply charging currents to these different battery systems. As the electrochemical nature of battery systems differ significantly, different battery chargers are necessary to address the different types of battery charging needs.
Many battery chargers today use a dual rate charge sequence in which the battery under charge is charged at a fast rate for a period of time, and then charged at a slower or "trickle" rate once the battery has reached a predetermined charge level. This particular charging regime has been particularly successful on Nickel-Cadmium (NiCd), and Nickel- metal hydride (NiMH) systems.
FIG. 1 illustrates a flow chart showing a prior art battery charging routine, particularly adapted for use with NiCd and NiMH secondary batteries systems. In step 12, the battery is inserted into the charging device. The charger in step 14 then determines the charge capacity of the battery by sensing the resistance of a capacity resistor (R.sub.c) which is located in the battery itself. Also, the temperature of the battery is sensed by sensing the resistance of a thermistor which is located inside the battery. If the measurements taken in step 14 are within pre-selected acceptable parameters, the charge sequence is enabled. If the values are not within the pre-selected parameters, the charger moves to step 18 and indicates a fault condition. The charger user is informed of the fault condition by, for example, observing an illuminated fault condition light emitting diode located on the charger.
Assuming no fault condition exists, the rapid charge sequence is initiated as illustrated in step 20. The temperature and voltage of the charged battery is monitored by a charge monitor device. Specifically, in step 22, the charger monitors the change in slope of the battery temperature to determine if the rapid charge rate is to be discontinued. Alternatively, or in addition, the charger could also monitor for changes in battery voltage. When the slope of the battery charging curve becomes negative, the battery is charged.
If battery temperature as determined by the charger reaches a predetermined value, the rapid charge sequence is terminated, in order to prevent overheating of the battery. The charger is then placed in a trickle-charged mode as illustrated in step 24. In the trickle-charge mode, the battery is charged at the rate of approximately C/10 to C/20, where "C" is the capacity of the battery. For example, if the battery has capacity of 1,000 maH at a C/10 charge rate, the charger would charge the battery using a current of 100 maH.
This charging regime does not, however, work well with Lithium ion cells. This is due to the fact that Lithium ion cells cannot withstand rapid charging techniques. Moreover, Lithium ion cells have demonstrated a propensity to explosively fail upon the application of excessive charging voltages. Lithium-ion cells have unique characteristics which make rapid charging difficult. For example, a voltage limit of approximately 4.2 V is imposed by all manufacturers for safety reasons. Further, continuous high current (i.e., greater than the cells 1C rate where "C" is the cell's capacity) causes metal lithium to plate onto the electrode. This permanently reduces the cell's capacity.
The recommended charging regime for Lithium ion cells is therefore a constant current/constant voltage approach, which, as recommended by manufacturers, takes 21/2 to 3 hours to complete. Further, conventional charging regimes for Lithium ion cells tend to heat the batteries, which in turn causes the battery's useful life to be shortened.
Accordingly, there exists a need for a method to efficiently, rapidly, and most importantly, safely, charge Lithium ion electrochemical cells.